1. Field of the Invention
The present invention relates to a discharge lamp that is preferably used as a light source for exposure in the manufacture of a liquid crystal display element, a color filter, a printed circuit board or a semiconductor element.
2. Description of the Related Art
In the case of a manufacturing process of a liquid crystal display element, a color filter, a printed circuit board and a semiconductor element and the like, a photolithography process is utilized for performing exposure of micro-images, and a short-arc type discharge lamp is used as a light source for exposure. Such a short-arc type discharge lamp is made such that there is provided a bulb having bulb-sealing sections jointly connected to both ends of a light emitting tube which encloses a light emitting space, and both an anode and a cathode are arranged opposite to each other within the light emitting tube of this bulb, and both inert gas and mercury or the like, for example, are hermetically filled therein.
In FIG. 3, a practical example of a conventional short-arc type discharge lamp will be described as follows, wherein a bulb 50 of the discharge lamp is made such that outward extending cylindrical sealing tubes 52 are jointly arranged at both ends of an elliptical spherical light emitting tube 51 enclosing a light emitting space S, and narrowed sections 52a provided such that the diameter of the formed sealing tube 52 is reduced at locations near the light emitting tube 51 of the sealing tubes 52 following the light emitting tube 51.
Within the light emitting tube 51 an anode 53 and a cathode 54 are arranged opposite, each of these electrodes is fixed to and supported by an electrode rod 55 extending from the sealing tube 52 to the light emitting tube 51 along an axis of the bulb. Within the sealing tubes 52 are arranged electrode rod holding cylinders 56 having cylindrical holes adapted to the outer diameters of the electrode rods 55 in a state in which the base ends of the electrode rods 55 are inserted therein, the outer circumferential surface of the electrode rod holding cylinder 56 adheres and is hermetically sealed to the inner surface of the narrowed section 52a. Each of sealing glass members 57 is comprised of an inner frustro-conical end 57a, the diameter of which is enlarged in the direction of the outer end, and a cylindrical barrel 57b following the inner end is arranged at an outer end of the electrode rod holding cylinder 56. The sealing glass member 57 is formed with a hole 57c having a bottom part extending from its outer end surface along an axial direction, and an outer lead rod 58 having an outer diameter adapted to its inner diameter is inserted into the hole 57c having the bottom part.
A plurality of belt-like metallic foils 60 are spaced apart from each other in a circumferential direction of the sealing glass member 57 and arranged at the outer circumferential surface of the sealing glass member 57 so as to extend from the inner end to the outer end of the sealing glass member 57, the inner end of each of the metallic foils 60 extends along the inner end surface of the sealing glass member 57 and is connected to the electrode rod 55, an outer end of each of the metallic foils 60 extends along the outer end surface of the sealing glass member 57 and is connected to the outer lead rod 58. In addition, the outer circumferential surface of the sealing glass member 57 adheres and is hermetically sealed to the inner surface of the sealing tube 52 through the metallic foils 60. At an outer end of the sealing glass member 57 is arranged a lead rod holding cylinder 59 having a cylindrical hole adapted to an outer diameter of the outer lead rod 58 in a state in which the outer lead rod 58 is being inserted, and the outer circumferential surface of the lead rod holding cylinder 59 adheres and is hermetically fixed to the inner surface of the sealing tube 52.
Such a discharge lamp as described above is normally lit in a vertical lighting process, i.e., the anode 53 and the cathode 54 are facing oppositely in a vertical direction in order to maintain a stable state of the arc formed between the anode 53 and the cathode 54 during lighting of the discharge lamp. However, in the case in which the discharge lamp is lit in such a way that the cathode 54 is positioned above the anode 53 in such a vertical lighting system, the direction in which mercury vapor flows from convection between the anode 53 and the cathode 54, and the direction in which thermo-ionic electrons discharged from the cathode 54 move towards the anode 53 are opposite from each other so that the motion of the thermo-ionic electrons is influenced by the convection of the mercury vapor with the result that there is a possibility of arc fluctuation.
Furthermore, in the case in which the discharge lamp is lit such that the anode 53 is positioned above the cathode 54, the direction in which the mercury vapor flows in a convection between the anode 53 and the cathode 54 and the direction in which the thermo-ionic electrons discharged from the cathode 54 move toward the anode 53 are made to be the same so that the motion of the thermo-ionic electrons is not influenced by the convection of the mercury vapor with the result that the state of the arc can be kept in a more stable condition. However, in the case in which the discharge lamp is lit such that the anode 53 is positioned above the cathode 54, there occurs numerous problems. For example, in a state in which the discharge lamp is not lit, almost all of the mercury hermetically filled within the light emitting space S of the bulb 50 is present in a state in which the mercury is condensed at the end part of the cathode 54 in the light emitting space S (on the inner end surface of the electrode rod holding cylinder 56). Then, when the discharge lamp is lit in such a state, since the temperature of heat generated at the cathode 54 is substantially low as compared with the temperature of heat generated at the anode 53, it is not possible to evaporate the mercury condensed at the end part of the cathode 54 in the light emitting space S within a short period of time after starting lighting of the discharge lamp resulting in that it requires a rather long period of time until the light emitting intensity of the radiated light becomes stable.
In particular, in recent years it has been required to provide a discharge lamp having a high light emitting intensity of radiation light, and in correspondence with this requirement there is a tendency that the amount of mercury filled in the bulb 50 is increased. Although in the case that such a discharge lamp is lit such that the anode 53 is positioned above the cathode 54 it becomes hard to completely evaporate the encapsulated mercury resulting in that radiated light having a desired light emitting intensity cannot be attained.
The present invention has been made with respect to the aforesaid circumstances, and it is an object of the present invention to provide a discharge lamp in which the state of the arc can be stably maintained and the light emitting intensity of the radiated light can be stabilized within a short period of time after starting the lighting operation.
The discharge lamp of the present invention includes a bulb having a light emitting tube enclosing a light emitting space and sealing tube sections arranged at both ends of the light emitting tube; an anode and a cathode oppositely arranged to face each other within the light emitting space of the bulb; and electrode rods bearing the anode and, respectively, cathode at their extreme ends extending from the sealing tube sections of the bulb toward the light emitting tube, and mercury being hermetically filled in the light emitting space of the bulb, characterized in that a thermal conductor plate is arranged at the end part of the cathode in the light emitting space of the bulb in a state in which it is being contacted with the electrode rod bearing the cathode.
In the case of the discharge lamp of the present invention, the electrode rod holding cylinder with its end surface being exposed to the light emitting space is arranged at a position of the bulb near the light emitting tube in the sealing tubes in a way in which the outer circumferential surface is adhered and sealed to the inner circumferential surface of the sealing tube, and the electrode rod bearing the cathode is inserted in the cylinder hole formed in the electrode rod holding cylinder, the thermal conductor plate is a disc having a hole of an inner diameter adapted to an outer diameter of the electrode rod in its central part and it has a smaller outer diameter than the outer diameter of the electrode rod holding cylinder, and it is preferable that the thermal conductor plate is arranged at one end surface exposed to the light emitting space in the electrode rod holding cylinder in a way such that the electrode rod is being fitted into the hole of the thermal conductor plate. In addition, in the case of the discharge lamp of the present invention, it is preferable that the discharge lamp is lit in a way such that the anode is positioned above the cathode.
If the discharge lamp having the aforesaid configuration is arranged in such a way that the anode is positioned above the cathode, almost all of the mercury filled in the light emitting space of the bulb is present such that the mercury is condensed on the thermal conductor plate arranged at the end part of the cathode in the light emitting space in a condition in which the discharge lamp is not lit. Thus, when the discharge lamp is lit in such a condition as described above, the heat generated at the cathode is transmitted to the thermal conductor plate through the electrode rods, and at the same time the thermal conductor plate is heated upon reception of the radiated heat (in particular, infrared rays) discharged from within the arc so that the mercury condensed on the thermal conductor plate is evaporated early resulting in that the light emitting intensity of the radiated light can be stabilized within a short period of time after starting the lighting of the lamp.